The invention relates to aqueous adhesive dispersions and to a process for producing multilayered paper, cardboard and carton having improved layer and ply strength, using said adhesive dispersions.
Multilayered papers are obtained from paper pulp mixtures of same or different composition by compressing (couching) the wet paper webs. Essentially, the quality of multilayered packaging papers or cartons is determined by the internal strength of these materials, the cohesion in the interface region between the individual plies of paper representing a weak point, however. Therefore, adhesive starch or starch derivatives are frequently used to improve the layer and ply strength. When exposed to major shear forces, heat or increased moisture during storage and further processing, however, stability may be lost even in products produced in this fashion, and the material may undergo breaking.
To date, native or modified starch in the form of an aqueous dispersion is sprayed as adhesive onto or between the paper plies, undergoing gelatinization in the drying section of the paper machine, thereby effecting bonding of the plies. Frequently, however, the layer and ply strength achieved in this manner is limited, particularly in those cases where the starch bonding undergoes complete or partial irreversible embrittlement during subsequent heat exposure. According to F. Andruchovici and R. Wilken, PTS-Verlag Munich, 1994, PTS-Forschungsbericht, PTS-FB 09/93, p. 19, the use of native starch is disadvantageous in that native starch can be employed with only low solids content due to its rheological characteristics (high viscosity), while the exclusive use of cationic starch is disadvantageous for cost reasons.
According to EP-B-310,078, an adhesive composition of stable viscosity on the basis of gelled particulate starch is used in the production of corrugated cardboard, which starch is employed together with a polymeric thickening agent of basic reaction, such as an acetone-formaldehyde resin, in the form of an aqueous dispersion.
A concentrated starch glue for the production of corrugated cardboard is known from DE-A-44 01 529, which includes an ionic surfactant to reduce its viscosity.
According to EP-B-415,385, surface and web strength and tear resistance of paper and cardboard are achieved by spraying the wet paper webs with a slurry of non-gelled starch grains having a gel temperature of 35-55xc2x0 C., previously obtained by heating a mixture of crude starch, urea, phosphoric acid or a phosphate at a temperature of 80-150xc2x0 C. for a period of from 0.5 to 5 hours.
Other paper sizing agents are known from EP-A-742,316, which agents include cationized starch preferably esterified with unsaturated dicarboxylic acids or anhydrides.
These well-known adhesives can only be produced with high technical input and have an adhesive effect that is insufficient for many applications.
Likewise, the use of polyacrylamide instead of starch as strengthening agent in spray coating, as described by K. Sato in Japan Tappi Journal 49, 318-323 (1995), No. 2, is inappropriate. Alternatively, the product is employed in the form of a solution which, however, not only remains on the surface of the paper web after spraying, but rather, penetrates therein.
Furthermore, a process for producing an abrasion-resistant decorative laminate is known from EP-B-555,093, wherein an aqueous slurry of aluminum oxide particles and a high molecular weight cationic retention agent such as polyacrylamide is coated on the uppermost wet layer of paper material and then subjected to further conventional paper manufacturing.
EP-B-606,431 describes a composition for coating cardboard, containing cationic starch and a vinyl acetate polymerization product which are also coated together with pigments. As is well-known, however, laminating of paper sheet materials involves other requirements as compared to the production of multilayered paper per se.
It was therefore the object to provide an adhesive dispersion that would enable easy production of multilayered paper with improved properties, particularly with enhanced layer and ply strength.
Said object is accomplished by providing the aqueous adhesive dispersions according to the invention, containing
A) 0.1-10 wt.-%, preferably 2-6 wt.-%, relative to the overall weight of the dispersion, of an adhesive polymer in dispersed form,
B) 0.1-100 parts by weight, preferably 0.1-20 parts by weight, more preferably 0.1-10 parts by weight, relative to 100 parts by weight of component A), of at least one anionic or cationic polyelectrolyte, and
C) 0-50 parts by weight, preferably 0.1-30 parts by weight, more preferably 0.1-20 parts by weight, relative to 100 parts by weight of component A), of at least one polyalkylene glycol having a molar mass ranging from 200 to 100,000 g/mol.
At least one natural or synthetic adhesive polymer, preferably a polysaccharide, an optionally modified polysaccharide, a protein, and/or a polyvinyl alcohol is used as component A), which polymer is present in dispersed form in said dispersion, i.e., has not been heated above its gelatinization temperature.
It is particularly preferred to use starch, modified starch, cellulose ethers such as carboxymethylcellulose, polyvinyl alcohols, alginates, proteins such as glutin, casein, and/or guar meal as adhesive polymers. However, starch and/or starch derivatives are preferred for economic reasons.
In this context, native starch, such as potato starch, wheat starch, corn starch, rice starch, pea starch and/or mixtures thereof, modified starches such as starch degradation products, particularly dextrins, chemically modified starches such as substituted starch derivatives and especially anionic starches such as dialdehyde starch, carboxystarch, and starch degraded by persulfate, anionic starch esters such as starch phosphoric acid mono- or di-esters, starch acetates and starch citrates, anionic starch ethers such as carboxymethylstarch, carboxymethyl-2-hydroxyethylstarch, and carboxymethyl-3-hydroxypropylstarch, and cationic starch derivatives such as N-containing starch ethers, particularly starches having primary or secondary amino or imino groups, and tertiary amino groups positively charged by protonation with acids, and quaternary ammonium groups are used.
The cationic or anionic polyelectrolytes to be used as component B) according to the invention are selected from the group of synthetic or optionally modified natural polymer products. Preferably, they are water-soluble or water-swellable cationic or anionic synthetic homo- or co-polymers of monoethylenically unsaturated monomers with acid groups which at least partially are present as salts, or their esters with di(C1-C2)alkylamino(C2-C6)alkylalcohols, or their amides with di(C1-C2)alkylamino(C2-C6)alkylamines as reported in EP-A-0,013,416 or in EP-A-0,113,038, each one being used in protonated or quaternized form, and optionally other monoethylenically unsaturated monomers.
These polyelectrolytes are remarkable in that they do not act as a flocculant for component A) below the gelatinization temperature thereof, but rather, form stable aqueous dispersions with A).
Preferably, homopolymers or copolymers of water-soluble, monoethylenically unsaturated vinyl compounds, such as acrylic acid and methacrylic acid esters of dialkylaminoalkylalcohols in protonated or quaternized form, such as dimethylaminoethyl acrylate, acrylic acid and methacrylic acid amides of dialkylaminoalkylamines, in protonated or quaternized form, such as acrylamidopropyltrimethylammonium chloride and/or acrylamidopropyltrimethylammonium methyl methosulfate can be used as cationically active polyelectrolytes.
Furthermore, Mannich bases of polymers containing acrylamide, polyvinylamines, polyamines such as diethyleneamine, dipropylenetriamine, triethylenetetramine, and polyalkylenepolyamines (polyethyleneimines) with molar masses of from 450 to 100,000 g/mol, preferably liquid polyalkylenepolyamines with molar masses of from 450 to 20,000 g/mol or solutions thereof in water are suitable as cationic polyelectrolytes.
In case the dispersions according to the invention do not include any component C, only those polymer products are possiblexe2x80x94when using polymers as cationic polyelectrolytesxe2x80x94that do not include any aldehyde or methylol groups as functional groups.
Preferably, homo- and/or copolymers of monoethylenically unsaturated carboxylic acids and sulfonic acids, such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid and/or the alkali, preferably sodium, potassium or ammonium salts thereof, vinylsulfonic acid, acrylamido- and methacrylamidoalkanesulfonic acids such as 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl methacrylate, and styrenesulfonic acid, and/or the alkali, preferably sodium or potassium or ammonium salts are suitable as anionic polyelectrolytes.
Furthermore, vinylphosphonic acid and styrenephosphonic acid, as well as the alkali salts, preferably the sodium or potassium or ammonium salts thereof are suitable.
The copolymers can be formed using the above-mentioned ionic monomers and non-ionogenic, water-soluble, monoethylenically unsaturated monomers such as acrylamide, methacrylamide, N-(C1-C2)alkylated (meth)acrylamides, as well as N-vinylamide, vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinylpyrrolidone. In addition, suitable water-soluble monomers are N-methylolacrylamide, N-methylolmethacrylamide, as well as N-methylol(meth)acrylamides partially or completely etherified with monohydric C1-C4 alcohols, and diallyldimethylammonium chloride.
Likewise, the copolymers may include limited amounts of sparingly water-soluble and/or water-insoluble monoethylenically unsaturated monomers such as (meth)-acrylic acid alkyl esters and vinyl acetate, as long as the solubility or swellability of the copolymers in water is retained.
Moreover, the polymers products can be produced using crosslinking monomers having at least two reactive sites, preferably diethylenically unsaturated monomers, so as to have swellability or only limited solubility in water, or they may consist of water-soluble and water-swellable polymers.
According to the invention, water-soluble or water-swellable amphiphilic copolymers produced of cationic and anionic monomers may also be used.
Furthermore, melamine-formaldehyde condensation products and urea-formaldehyde condensation products in anionic and cationic modification, polyamidoamine-epichlorohydrin resins, e.g. the reaction products of adipic acid with dieethylenetriamine, or of similar amines and epichlorohydrin are suitable as component B).
The polyelectrolytes used according to the invention can be mixed with the adhesive polymers as a powder, dispersion or solution, preferably as an aqueous solution.
The weight ratio of the amounts of components A) and B), relative to the total amount of components A) and B), preferably ranges from 4:1 to 25:1, more preferably from 10:1 to 25:1, with a range of from 14:1 to 20:1 being particularly preferred.
Furthermore, the adhesive dispersions of the invention may additionally include other additives such as surfactants, wet-strength agents, sizing agents, hygroscopic agents, and pigments.
The adhesive dispersions are produced by combining the polymer and polyelectrolyte components, the components being placed in a vessel in any order, in portions or continuously in an aqueous phase, preferably as an aqueous dispersion, mixed by stirring or pumping, and optionally heated below the gelatinization temperature of component A). The adhesive polymer A) is heated at a temperature below the gelatinization temperature, namely, at 30-55xc2x0 C., preferably 35-50xc2x0 C., more preferably 40-45xc2x0 C., and simultaneously or subsequently added with a solution or dispersion of the polyelectrolyte of optionally the same or optionally the opposite ionogenicity. optionally, the dispersion can be heated to the maximum temperature below the gelatinization temperature for a certain period of time.
The addition of polyalkylene glycol to the dispersion can be effected as such or in the form of an aqueous solution of the polyalkylene glycols prior to or subsequent to adding component A) or preferably subsequent to adding component B). Heating of the dispersion, particularly the duration thereof, and the pH value of the aqueous dispersion can be determined by a person skilled in the art using simple tests depending on the raw materials used and the equipment-related conditions present and are selected in such a way that the gelatinization temperature of the components will not be exceeded. By adding the polyelectrolytes B) to component A) and heating the dispersion, the condition of the homogeneous dispersions is retained.
Heating of the dispersion may also be effected in such a way that the components are heated one by one or placed in preheated water.
The production of multilayered paper is well-known and is effected using paper machines equipped with multilayer screen sections, on which multiple paper webs are formed. To this end, rotary screens arranged in a well-known fashion, rotary screen molds, or an endless wire section as sheet-forming system, consisting of e.g. 2-6 endless wires, are used. Thereafter, the paper webs formed on the screen sections are treated in wet condition with the adhesive dispersions of the invention, the adhesive being coated onto at least one paper web, the webs are contacted and subsequently combined to form a thicker web. When subsequently passing the drying section, final sizing of the paper webs is effected using the adhesive dispersion.
Coating the adhesive dispersion onto at least one of the wet paper webs can be performed using the well-known coating techniques as reported e.g. in F. Andruchovici and R. Wilken, PTS-Verlag Munich, 1994, PTS-Forschungsbericht, PTS-FB 09/93, p. 16. According to the invention, in particular, the spraying technique is preferred over concurrent or countercurrent feed roll coating where wetting of the paper web can be effected over the entire surface, or limited e.g. by templates. To achieve sufficient sprayability of the adhesive dispersion, water-soluble polymers having a molecular weight m.w. (g/mol) of from 1000 to 2 millions, preferably from 1000 to 100,000, and more preferably from 1000 to 50,000 are preferably used as component B). Appropriate polymer products are obtained in a well-known fashion by adjusting the polymerization conditions, using suitable initiator and modifier combinations. Furthermore, it is possible to start off with high molecular weight polymer products and subject same to mechanical degradation, e.g. by shearing in aqueous solution, prior to combining with component A).
The adhesive dispersions according to the invention are remarkable for their excellent shelf-life when dispersed in aqueous phase.
When coating the adhesive dispersions of the invention, the dispersed portions precipitate on the surface or in the upper fibrous region of the paper web. The combined wet paper webs then pass the press section and subsequently the drying section of the paper machine where bonding of the paper webs to be treated is effected under the conditions present therein to form a multilayered paper.
Therefore, the present invention is also directed to methods of producing multilayered papers and other sheet materials comprised of paper material and made up of at least 2 paper webs, using the adhesive dispersions according to the invention, particularly in manufacturing packaging paper, corrugated cardboard, multi-web solid cardboard, carton, and multilayered test liner, as well as base paper for wallpapers, and other multilayered special papers remarkable for their enhanced stability and strength, particularly with respect to layer and ply strength.
Preferably, from 1 to 10 wt.-%, more preferably from 4 to 8 wt.-%, relative to the paper production, of the aqueous dispersion according to the invention is used.
Without intending to be limiting, the invention will be described in more detail with reference to the following Examples.
The ply strength is the measured value of the strength between an outer layer and the remaining other layers of the paper.
The layer strength is the measured value in the center of a multilayered paper with symmetrical structure.
The measurement readings are obtained by separating and subsequent pulling apart the paper layers, using a tensile strength tester in accordance with ASTM F 904-91.